This technique is often known as Click 2 Call, or Click 2 Dial and a number of solutions exist which enable you to click on a contacts telephone number either in your Email application or on a website and make your phone call them.
MAC Users can download a script from http://www.gradwell.net/downloads/VOIPPhone.scpt, this script has been written by a Gradwell customer and uses our Click 2 Call API. Instructions can be found at the beginning of the file on how to integrate it into your system.
If you use the Trixbox CE (Community Edition) product, then you can initiate phone calls using HUD Lite, further information can be found at http://www.hudlite.org/
If you use SNOM SIP Phones, then a free application can be downloaded from SNOM which integrates with Microsoft Outlook, please see http://wiki.snom.com/Outlook_Add_In for further details (please note that this is no longer developed by snom and will only work on newer versions of Windows and Outlook, nor newer snom firmware versions)
X-Lite and Eyebeam softphones can import Outlook contacts which then populates the local address book, please see http://www.counterpath.com/x-lite.html
Custom solutions can be written by taking advantage of TAPI (Telephony Application Programming Interface), further information can be found at the websites linked to below: